Getting new products to the market faster than one's competition is recognized as a key to gaining a large market share. Thus, there is an incentive to speed up every step of new product development. One area of product development having a significant impact on overall market timing is the making of product and package prototype for market testing. Such testing usually requires multiple look-like, feel-like, and function-like prototypes for consumers to examine or use.
Where production quantities are needed, molding is the normal way of producing parts. Production molding typically involves plastic parts made in very expensive, multiple cavity, steel molds. For example, most bottles are blow-molded and most bottle closures are injection molded. It usually takes large production quantities to justify the cost of a production mold. For market testing, on the otherhand, only a few hundred parts may be needed. However, it is often necessary to mold prototype parts so that they have the same characteristics as production parts.
How to rapidly obtain molded prototype parts is therefore the challenge. Some solutions already exist for rapidly maling prototype part molds from which a small quantity of parts can be cast. For example, refractory powders and a thermoplastic binder can be combined under heat in a flexible rubber pattern. This process forms a green article, which is then heated further to melt out the binder. Infiltrating the resulting porous article with a molten, low melting point metal forms a mold of high density which is free of machined surfaces. The disadvantage of this process is that the powders are sintered in order to hold the mold together after the binder is removed. Sinteing causes particles to occupy less space than they would occupy unsintered. Thus, sintering shrinkage influences the accuracy of the mold and the parts made therefrom.
A siftered metal article having channels, such as for cooling fluid, may be formed by combining copper wires with sintering powders. Upon application of the sintering temperature, the wires melt and are absorbed into the pores of the sintered particles to form channels. Other processes in the art involve metal particles which are sintered together to form a matrix into which an infiltrating metal can be solidified. The sintering process causes particles to change their spacing somewhat, leading to inaccuracies in the metal infiltrated part dimensions.
Another method for rapidly prototyping parts is investment casting, using patterns generated by rapid prototyping systems instead of traditional injection molded wax patterns. An example of such a pattern is a QuickCast.TM. pattern, which is a trademark of 3D Systems, Inc. of Valencia, Calif. A hollow plastic pattern is coated with a thin ceramic shell, usually by a dipping process. The plastic is burned out of the ceramic shell leaving mininal amounts of ash residue behind. Molten metal is then poured into the ceramic shell to cast a metal part or a metal mold for a plastic part. Because the shell has only a small hole for admitting molten metal, it is difficult to inspect the critical surfaces for ash residue. Any ash reman on a critical Ice will potentially ruin the metal casting. The molten metal cools and shrinks such that critical surfaces are not reproduced accurately. The larger the parts, the greater the inaccuracy.
Improvements to the investment casting process utilize a ceramic shell which is created around a pattern by pouring a ceramic slurry and a binder that is chemically controlled to provide for precise setting of the ceramic shell. This is an improvement to the investment casting process because shell-making is faster. However, investment casting is still limited to small size molds where molten metal shrinkage does not exaggerate ares.
An improved method of constructing a fully dense mold is disclosed in U.S. Pat. No. 5,507,336 issued to Tobin, April, 1996. The method comprises placing a pattern within a tube which has a melting point greater than that of the infiltration material which will be used in making the metal mold. A ceramic member is cast between the pattern surf and the open end of the tube to transfer the critical pattern surfaces to the ceramic member. The ceramic surfaces are inverse to the pattern surfaces. The pattern is burned out and the ceramic surfaces remains in the tube. The ceramic is then covered with metal powder and an infiltration material from the other end of the tube, and the tube is placed in a furnace to form the metal part over the ceramic surfing. The metal pot has surfaces inverse to the ceramic surface. A met mold results when the ceramic piece is removed. The metal mold has the same shape as the pattern, and is useful for molding plastic parts having an inverse shape. This is an ideal proccss for parts having exterior critical surfces.
Tobin's process destroys the pattem from which the ceramic mold is created. A process for quickly forming a ceramic mold pattern which does not destroy the pattern, but which is accurate, is needed. Also, it is often necessary to provide a mating metal mold for plastic part molding. In order to do this, the metal mold may require a shape which is the inverse of the pattern. Thus, the ceramic mold needs to have the same shape as the pattern, and therefore requires an intermediate mold be produced between the ceramic mold and the pattern. As with Tobin's earlier process, any ceramic mold should not be contaminated on its surface so that the resulting meta mold is accurate.
In order to avoid destroying the pattern, it is desirable to use an intermediate mold made of a material which can be discarded or reused as needed to transfer the critical pattern surces to the ceramic mold. Wax and silicone rubbers have been used for these purposes. Wax (which is heat reversible) has the disadvantage of being brittle and when removed from the pattern can cause small pieces to break off especially where undercuts and thin features are involved. It also can expand and crack the ceramic when heated. Silicone rubbers need to be cured, and when the ceramic releases heat as it "sets", the silicone rubber can distort and cause inaccuries to develop in the ceramic pattern. Also, silicone rubber has to be removed from the pattern by air injection or other means which forces the silicone from the ceramic. This can cause the ceramic mold to break especially where undercuts and thin features are involved.
What has been missing is a method which avoids sintering for rapidly making accurate metal molds primarily for injection molding purposes, independent of part size, which enables a retatively large number of plastic parts to be molded therefrom.
It is therefore an object of this invention to provide a process for making a metal mold having an inverse shape to a pattern, which produces accurate reproductions of a pattern of any size, within a tolerance of .+-.0.005 inches.
It is also an object of this invention to provide a process which uses an elastic, heat reversible material to make an inverse intermediate mold of a pattern and which is not distorted during the forming of a ceramic mold therefrom, but which can be removed easily from the ceramic mold without destroying the delicate features of the ceramic mold.
These and other objects will be evident from the description herein.